Differential modulation is considered an attractive form of technology, as it obviates the need for a pilot signal at the transmitter, for increased spectrum efficiency, and does not require channel information at the receiver.
Recent studies have developed a technique, known as differential space-time block coding, which applies differential modulation to orthogonal space-time block coding for multi-antennas. In a small terminal, however, it may be difficult to install several antennas, due to size and space constraints. To overcome such difficulties, the distributed diversity (relay diversity) technique has been recently developed.
This involves obtaining diversity gain by processing the signals of the transmitter end and the relay simultaneously. However, much of the research on relays assumes that the channel information is known.
In spite of the research on differential modulation for relays, the research on differential space-time block coding has been limited only to basic space-time block codes, and the research has not much focused on relay systems using differential orthogonal space-time block coding, due to the high level of complexity required at the receiver end.